Homogenous charge compression ignition (HCCI) is a mode of combustion offering the potential for significant improvements in efficiency and substantial reduction in emissions. HCCI further offers the potential for meeting or exceeding the more restrictive emissions regulations expected in the near future. HCCI engines typically initiate combustion using a thoroughly pre-mixed fuel/air mixture, which may be mixed in the intake port or the cylinder. Within the cylinder, the density and temperature of the fuel/air mixture is increased through compression until ignition occurs. As the ignition occurs at several locations at a time, the fuel/air mixture burns nearly simultaneously, which greatly reduces NOx and PM emissions compared to traditional combustion engines such as the diesel engine. The HCCI engines also realize other benefits including enhanced fuel economy due to their higher compression ratios and the absence of throttling.
It should be understood that the term “HCCI” as used herein is intended to include any engine condition for which completely homogeneous fuel-air mixing does not necessarily occur, yet significant fuel-air mixing still takes place, e.g., so-called pre-mixed charge compression ignition (PCCI).
Unfortunately, HCCI engines are difficult to control due to the extremely rapid combustion and absence of a triggering ignition event. At higher temperatures, there is a tendency for the pre-mixed air/fuel mixture to combust rapidly. If the combustion is especially rapid, high rates of pressure rise can cause excessive noise and potential engine damage. The traditional measures available in gasoline and diesel engines for triggering ignitions are not particularly useful for controlling ignition timing and combustion in an HCCI engine.
Other challenges facing HCCI engines include lack of universal, yet practical, measures of ignition quality of HCCI fuels, and excessive particulate/smoke emissions during operation on diesel boiling range fuels, especially at high engine loads. In addition, HCCI engines operate at high air/fuel ratios and/or high exhaust gas recirculation (EGR) rates for the purpose of controlling combustion phasing, peak cylinder pressure, rate of cylinder pressure rise and/or NOx emissions. This restricts the amount of fuel that can be burned in the course of an engine cycle and thus limits the maximum achievable engine loads. For example, HCCI engines operated on a typical 45 cetane number US diesel fuel can typically produce, at most, only ⅓ of the load attainable by comparable diesel engines, if the comparison is made at the same diesel-like compression ratio.
Studies have shown that the fuels formulated for combustion in an engine under HCCI conditions require an ignition quality that is significantly different from gasoline and diesel fuels currently on the market. Producing, distributing and marketing a completely separate fuel exclusively for HCCI applications presents a significant economic investment and burden on fuel companies.
Accordingly, there is a need for a fuel composition, and a method for producing the same particularly suited for combustion under HCCI conditions. There is a need for a fuel composition and method for producing the same, capable of at least meeting the ignition quality requirements of optimum HCCI fuel. There is a further need for a fuel composition and method for producing the same that utilizes existing infrastructure for storing and supplying a fuel optimized for HCCI applications.